1. Technical Field
The present invention relates to a surface acoustic wave (SAW) device such as a resonator, a filter, and an oscillator, and to a method of manufacturing the same. In addition, the present invention relates to an IC card and a mobile electronic apparatus having the SAW device.
2. Related Art
In the related art, SAW devices having SAW elements including IDTs (comb-shaped transducers: Inter Digital Transducers) constructed with alternate finger electrodes formed on a surface of a piezoelectric substrate and reflectors and using surface acoustic waves excited from the IDTs have been widely used for various electronic apparatuses. In particular, in addition to high frequency performance and high precision coping with high speed information communication, thinner SAW devices have been required in for small-sized, thin information apparatuses such as IC cards.
In general, a SAW device in the related art is constructed by hermetically sealing SAW elements in a package which is formed by bonding a metal lid to a base made of a ceramic material with seam-welding. In the SAW element, bonding pads of an IDT formed on a surface of a piezoelectric substrate are electrically connected to connection terminals of the package by using bonding wires (for example, see JP-A-2002-16476 and JP-A-2003-87072). In such a structure, the number of parts is quite large, and the construction is too complicated. In addition, in order to provide the bonding wires, a large space is needed in the package, so that there is a limitation on implementing a small-sized thin SAW device.
Therefore, there has been proposed an electronic part constructed by accommodating the SAW device in a space formed between upper and lower cases, electrically bonding input/output electrodes of the SAW device to second connection members of an outer surface of the case with first connection members disposed in through-holes of the upper case, and sealing the through-holes with the first or second connection members (for example, see JP-A-7-86867). As a result, by simplifying the sealing structure of the through-holes, a simple construction and a small size can be obtained.
In addition, there has been proposed a double-layered surface acoustic wave apparatus including a surface acoustic wave element having IDT electrodes, extraction electrodes, and positive electrode bonding portions on a substrate and a cover plate constructed with a glass plate having through-holes provided with external electrodes, wherein the extraction electrodes and the external electrodes are electrically connected, and wherein the cover plate and the positive bonding portion are bonded to seal the IDT electrodes (for example, see JP-A-8-213874). According to this construction, since the base and the bonding wires mounted on the SAW element are omitted, the entire construction can be simplified, and a small-sized thin product can be obtained. In addition, production cost can be reduced, and frequency characteristics can be stabilized.
On the other hand, in order to improve a high frequency performance of the SAW device, spaces between the alternate finger electrodes must be shortened. In addition, if extrinsic material such as electrode material debris generated during manufacturing processes remains in the space between alternate finger electrodes, a short-circuit may occur. In order to solve this problem, there has been proposed a SAW apparatus capable of preventing short-circuiting between the electrodes without a decrease in Q value by forming a protective film constructed with an oxide film (alumina) with a positive electrode oxidation processes on a surface of an IDT (for example,see JP-A-8-130433).
In the double-layered SAW apparatus disclosed in JP-A-8-213874, the SAW element and the cover plate are bonded by aligning and closely adhering the extraction electrodes and the through-holes at the positive electrode bonding portion, and thereafter, the external electrodes are formed at the through-holes and peripheral portions thereof and are electrically connected to the excitation source by using a sputtering process in a vacuum. In general, since the extraction electrodes are made of aluminum, the surface thereof is naturally oxidized due to air contact. Therefore, after the positive electrode bonding between the SAW element and the cover plate, although the external electrodes are formed in a vacuum, sufficient electrical conduction to the extraction electrodes cannot be ensured.
In addition, the sealing of the through-holes is performed by forming the extraction electrodes and the electrode of the positive electrode bonding portion with the same film thickness so as not to generate gaps between the extraction electrodes and the through-holes and forming the external extrudes on inner sides of the through-holes. However, in practice, it is difficult to surely hermetically seal the gaps between the inner side openings of the through-holes and the extraction electrodes with the electrode films by using the sputtering process.
In addition, since the double-layered SAW device is a surface mounting type, the external electrodes formed on a surface of the cover plate are directly adhered on a surface of a print board or the like with a solder, an adhesive, or the like. However, as the size of the device is smaller and smaller, the mounting area is reduced. Therefore, it may be difficult to mount the SAW device in a good state and obtain sufficient adhesive strength after the mounting thereof, so that sufficient mechanical strength against an external force such as bending cannot be secured.